Abstract

Micro-Raman spectroscopy enables precise non-destructive analyses of CO2 density in a very small volume. By applying this method to CO2 fluid inclusions in minerals, a contrast is apparent in the CO2 density specific to mineral species in a mantle-derived spinel lherzolite xenolith entrained by ascending magma. The rock investigated in this study comprises four mineral species: olivine, orthopyroxene (opx), clinopyroxene (cpx), and chromianspinel (spinel). The CO2 densities in the fluid inclusions in these minerals are 1.006–1.035, 1.148–1.154, 1.150–1.154 and 1.189–1.194 g/cm3, respectively. During transport of the rock by magma and subsequent cooling, the CO2 fluid inclusions change their volume due to both elastic and plastic properties of the host mineral, which are sensitive to differential pressure between internal pressure of the CO2 fluid inclusion, and stress in surrounding crystal lattice. We tested the possibility that the volume of CO2 fluid inclusion changes by the differential pressure. Existing models dealing with the volume change of fluid inclusion in response to deformation of host mineral do not explain the density gradation, particularly that between pyroxenes and spinel. We propose that combination of precise determination of fluid density in mantle-derived minerals and observation of microstructure in the host mineral provides a deep insight into the deformation mechanism of natural minerals under high differential pressure and temperature.